The present invention relates to methods of disinfecting and stabilizing organic waste so as to produce a stable, granular bio-mineral product, which has reduced levels of harmful pathogens. The methods include intimately mixing organic waste with one or more mineral by-products to form a mixture having a pH less than about 9, and heating and drying the mixture to produce a stable, granular bio-mineral product. Suitable mineral by-products according to the present invention include those that will result in the mixture having a pH below about 9, thereby minimizing the production of odor-causing gases, such as ammonia and methyl amines from the organic waste.
The present invention also relates to a stable, granular bio-mineral product produced by these methods. The invention further relates to fertilizers, soil amendments and soil substitutes that include these stable, granular bio-mineral products.
Various techniques have been developed for the purpose of sterilizing or decontaminating biological sludges and wastes. In 1993 the U.S. Environmental Protection Agency promulgated rules for the treatment and management of municipal sewage sludge (EPA, 1993). These rules set standards for pathogen destruction (disinfection), vector attraction reduction (VAR), and metal contaminant reduction in sewage sludge. The disinfection standards are separated into two categories, Class B in which sludges are treated to partially destroy pathogens, and Class A where pathogenic bacteria, enteric viruses and helminth parasites are reduced to near detection limits.
Processes previously approved by EPA as Class A disinfection processes include: thermal treatment, based on a prescribed time-temperature relationship; advanced alkaline stabilization with accelerated drying, combining raising a pH above 12 for 72 hours, heating to greater than 52xc2x0 C. for 12 hours, and producing solids greater than 50%; composting; heat drying; heat treatment of liquid sludge; thermophilic aerobic digestion; beta ray irradiation; gamma ray irradiation; pasteurization (temperature greater than 70xc2x0 C. for at least 30 minutes); a combination of a pH reaching at least 12 and pasteurization; and several advanced digestion processes (EPA, 1999).
These disinfection processes may be classified as follows: (1) heat; (2) high pH (reaching a pH over 11 or 12 during the process); (3) digestion; (4) irradiation; or (5) drying processes, or a combination of one or more of these processes.
Heating to a sufficient temperature for a proscribed period of time is an effective disinfectant method, and complete sterilization may be achieved at temperatures below 100xc2x0 C. for a few minutes. The U.S. Environmental Protection Agency established a sliding time-temperature scale based on the equation:
D=131,700,000/100.1400t 
where D=days and t=temperature (xc2x0 C.) (EPA, 1999), in one of its methods of achieving Class A disinfection.
Because temperature is the sole disinfectant under this alternative, the EPA has required that the process demonstrate that every particle of sludge be exposed to the required temperature for the required time.
The use of chemical additives such as acids, alkalis, and combinations of acids and alkalis with electric heat or heated air to disinfect by heating has been described, for example, in U.S. Pat. Nos. 4,079,003; 4,902,431; 5,196,043; 5,346,616; 5,422,015; 5,525,239; 5,746,006; 5,853,590 and 6,056,880. These methods rely exclusively on high heat (185xc2x0 F.) for disinfection. They are disadvantageous in that the high temperature is high enough for long enough to destroy most if not all beneficial microorganisms. The use of direct and indirect dryers to dry materials such as sewage sludge is known.
The importance of achieving disinfection at the lowest possible temperatures so as to preserve viable populations of non-pathogenic microorganisms and thereby enhance product stability has also been described, for example, in U.S. Pat. Nos. 4,781,842; 4,902,431; 5,135,664; 5,275,733; and 5,417,861.
Examples of Class A disinfection processes relating to high pH (that is, reaching a pH of 12 or higher at some point in the process) as a specific disinfectant are set forth, for example, in U.S. Pat. Nos. 3,476,683; 4,079,003; 4,554,002; 4,781,842; 4,902,431; 4,997,572; 5,135,664; 5,196,043; 5,229,011; 5,277,826; 5,853,590. Although high pH is a proven disinfectant, a high pH product has problems associated with odor-causing gaseous emissions due to alkaline reactions with the sludge. Among the noxious gases are ammonia and di- and trimethyl amines.
A traditional method of stabilizing organic wastes against odor generation and attraction of disease vectors such as flies, is to add alkali and raise pH between 11 and 12. EPA recognizes high pH as a means of achieving VAR under 40 C.F.R. part 503 (EPA, 1999). In this method, the sludge pH must be held at pH 12 for 2 hours and subsequently at pH 11.5 for 22 hours. This is a temporary stabilization process if insufficient alkali is added to maintain the pH of the sludge until it can be incorporated into the soil. Likewise, EPA recognizes VAR by high solids but this form of stabilization alone is inadequate if the material is wetted.
The art has used mineral by-products in stabilizing semi-solid, odorous organic wastes through bulk drying, odor absorption, and granulation, see for example, U.S. Pat. Nos. 3,877,920 and 4,554,002. Mineral materials including sand, diatomaceous earth, perlite, and various mineral reagent powders have been used in conjunction with fluidized heating, drying and burning of sludges and oily wastes. See for example, U.S. Pat. Nos. 4,159,682, 4,787,323, 4,970,803, 5,490,907.
Burnham and Striebig have disclosed a method to combine acid, alkali and external heat to pasteurize and chemically modify sewage sludge. The purpose of the acid and alkali addition in these methods is to adjust pH to a desirable range (i.e., over 12). No by-products are used in these processes, only virgin acid and agricultural limestone.
Digestion methods, which are well known in the art, are only applicable to liquid wastes. Digestion is also disadvantageous because the digestion process is susceptible to disturbances of flow, nutrient loadings, temperature, chemical content, accumulated sludge levels and other influences; digestion requires long retention times in large tanks; and close supervision of the process by skilled operators is often required for acceptable performance, although such supervision is no guarantee of a good outcome.
Irradiation methods disinfect by directly destroying pathogen cells. These methods are not used in practice.
An efficient method of disinfecting and stabilizing organic waste without destroying beneficial microorganisms, as in previously known heating methods, is desirable. Additionally, a method resulting in a product that does not produce significant amounts of noxious odors caused by the products of known high pH methods is also desirable.
The present invention relates to methods of disinfecting and stabilizing organic wastes by intimately mixing organic waste with mineral by-products, and heating and drying the bio-mineral mixture having a pH of less than about 9. The one or more mineral by-products preferably include a mineral by-product having a pH of less than about 9 or a mixture of mineral by-products having a combined pH of less than about 9. The present methods are advantageous over previous methods in that they are efficient, permit the survival of beneficial microorganisms and provide a product that is granular, relatively free of high pH-causing odors, stable, and may be used as a fertilizer, soil amendment or soil substitute.
The present invention also relates to stable, granular bio-mineral products produced by the methods described herein and to fertilizers, soil amendments and soil substitutes that include these stable, granular bio-mineral products.
The present invention further relates to systems for disinfecting and stabilizing organic waste and for making a stable, granular bio-mineral product. These systems include a means for intimately mixing organic waste and one or more mineral by-products, where the mixture has a pH less than about 9. The systems further include heating means and drying means to heat and dry the mixture.
In some embodiments of the present invention, at least a portion of the heat for heating, drying, disinfecting and/or stabilizing the mixture of organic waste and mineral by-product(s) is provided by residual heat in hot fly ash or other hot mineral by-products.